The present invention relates generally to a writing medium reservoir, and a writing instrument containing same, in which the writing medium is readily visible to the user, and which also advantageously exhibits substantially clear drain of the writing medium through the reservoir to the writing tip. The present invention also relates to a method of making such a writing medium reservoir, and a writing instrument containing same. In addition, the present invention relates to a multi-layer tubular structure, as well as a method of making same, in which the liquid medium therein is readily visible to the user and which tubular structure also advantageously exhibits substantially clear drain of the liquid medium through the tubular structure.
A writing instrument contains a writing medium reservoir, e.g., an ink tube, charged with a selected amount of a writing medium, e.g., ink, which is depleted during the use of the writing instrument. Occasionally, the writing medium leaves a residue on the surface of the reservoir. It is desirable to reduce, if not to eliminate, the amount of such residue, so that as little writing medium as possible is left behind as residue (and thus wasted).
Additionally, it is often desirable to have a writing medium reservoir that is transparent or translucent so that the writing medium therein can be observed. It is also desirable to have as little residue as possible so that the view into the reservoir is substantially unobstructed and/or so that the content of writing medium in the reservoir can be readily determined. When the writing medium reservoir is transparent or translucent, a user can see the level of the writing medium within the reservoir.
If little or no residue remains on the interior surface of the writing medium reservoir as the writing medium level changes, then the writing medium reservoir is said to exhibit clear drain. As may be appreciated, clear drain is desirable in a transparent or translucent writing medium reservoir so that the user can accurately determine the amount of writing medium remaining in the reservoir.
When a material exhibits clear drain with respect to a liquid (regardless of its viscosity), such as a writing medium in this case, the material usually also exhibits anti-wetting properties with respect to that liquid, which typically means that the material has a significantly lower surface energy than that of the liquid. The difference in surface energies between the relatively solid material and the relatively liquid material (e.g., more formally, the difference between the surface tension of the writing medium and the surface energy of the material forming the interior surface of the reservoir) is manifested by the liquid preferring to associate with itself rather than with the material surface (e.g., by spreading out over the surface). Generally, but not necessarily universally, a material that exhibits anti-wetting properties with respect to a given liquid will also exhibit effective barrier properties against diffusion, sorption, or permeation of the given liquid into or through the material, e.g., against evaporation of the given liquid. Effective barrier properties of anti-wetting materials in writing medium reservoirs are especially desirable when the writing medium has a relatively volatile liquid base (e.g., such as water, acetone, butyl acetate, etc.), in order to prevent, control, or inhibit (hereinafter xe2x80x9ccontrol,xe2x80x9d without any intent to unduly limit) diffusive or permeative evaporation thereof through the reservoir.
While there are examples in the prior art of materials for use in writing medium reservoirs exhibiting anti-wetting properties or clear drain to some extent, none of these materials has been disclosed to be transparent or translucent materials. Transparent or translucent materials may have been independently disclosed for use in writing medium reservoirs, but the types of materials that can be easily fabricated into transparent or translucent parts are generally not the same types of materials that exhibit anti-wetting properties or clear drain with respect to typical writing media. While the material properties of transparency/translucency and clear drain are not diametrically opposed, the prior art has not successfully combined these properties in writing medium reservoirs.
In fact, only relatively recently has a polymeric fluorinated hydrocarbon material having a relatively low surface energy, which is normally fabricated as an opaque part, been fabricated as a thin sheet allegedly having transparent or translucent properties. Even so, significant effort and careful attention to fabrication parameters were required to attain the transparent or translucent properties in this thin sheet. Such a transparent material has not been suggested to be fabricated into a tubular shape, no less into a shape useful as a writing medium reservoir of a writing instrument.
One aspect of the present invention relates to tubular structures such as writing medium reservoirs exhibiting anti-wetting properties (e.g., at least partially exhibiting clear drain) with respect to a fluid such as a writing medium. Exemplary writing medium reservoirs formed in accordance with the principles of the present invention may be used in writing instruments including, but not limited to, ball point pens, rollerball pens, free ink pens, pens using gel ink, retractable tip pens, etc.
Clear drain is particularly beneficial in transparent or translucent writing medium reservoirs, so that the user can accurately observe the level of writing medium therein. However, even when a writing medium reservoir is not transparent or translucent, the anti-wetting properties of the writing medium reservoir are beneficial to reduce wastage of writing medium.
Anti-wetting properties can be measured, for example, by determining the difference between the surface tension of the fluid and the surface energy of the material forming the interior surface of the tubular structure. In cases where the surface tension of the fluid is equal to or less than the surface energy of the material forming the interior surface of the tubular structure (i.e., when the difference is a negative number), wetting of the tubular structure interior surface material with the fluid will usually occur. Even in certain cases where the surface tension of the fluid is slightly greater than the surface energy of the material forming the interior surface of the tubular structure (i.e., where the difference between those quantities is a relatively small positive number), there may still be sufficient wetting of the tubular structure interior surface by the fluid such that substantially clear drain is not exhibited. In cases where the surface tension of the fluid is more than slightly greater than the surface energy of the material forming the interior surface of the tubular structure (i.e., where the difference between those quantities is more than a relatively small positive number), the tubular structure interior surface generally has sufficient anti-wetting properties to exhibit substantially clear drain of the fluid. It is conceivable, however, in certain select cases where the surface tension of the fluid is significantly greater than the surface energy of the material forming the interior surface of the tubular structure (i.e., where the difference between those quantities is a relatively large positive number), that the fluid, instead of exhibiting substantially clear drain from the tubular structure interior surface, can have such strong molecular interactions with itself and can exhibit such non-wetting character with respect to the tubular structure interior surface material that it beads up on the tubular structure interior surface material and thus does not exhibit substantially clear drain properties.
In accordance with another aspect of the present invention, a writing medium reservoir may be formed from or may contain a material that is advantageously transparent or translucent (hereinafter xe2x80x9ctranslucent,xe2x80x9d for simplicity and without any intent to limit). Such transparency or translucency (hereinafter xe2x80x9ctranslucency,xe2x80x9d for simplicity and without any intent to limit) may be beneficial in conjunction with the above-described tubular structure that has anti-wetting properties, although the combination of both features is not required.
However, it is particularly beneficial to have a tubular structure such as a fluid reservoir or conduit that not only is translucent, but also exhibits clear drain. In accordance with the principles of the present invention, any translucent, and preferably optically transparent, writing medium reservoir, made of any desired material may be provided with an inner layer of a translucent, and also preferably optically transparent, relatively low surface energy polymer or copolymer to result in clear drain in a writing medium reservoir permitting external observation of the writing medium therein. In this manner, a wide range of materials may be used to form the writing medium reservoir, and only a small amount of relatively low surface energy polymer is necessary.